1. Field of the Invention
The present invention relates to a static relay, as well as to the application thereof to a bipolar inverter or to a using circuit in which a current flows in a random direction.
2. Discussion of Background
FIG. 1 diagrammatically shows a known circuit making it possible to regulate the intensity value of a current flowing in a load circuit or load (G,Z), by means of a current amplifier stage 1 comprising a current input and output 2, 3 respectively. The output of amplifier 1 is connected to a first terminal of load Z. The current is supplied by a generator G having a first supply terminal 4 connected to the amplifier input 2 and a second supply terminal 5 connected to a second input of load Z. This known amplifier (DARLINGTON circuit) generally comprises a power amplifier stage 6 constituted by a transistor T1, e.g. of the npn type, whose collector is connected to the amplifier input 2 and whose emitter is connected to the amplifier output 3. This amplifier also comprises a stage 7 for controlling transistor T1 of the power stage. This bipolar control transistor is of the same type as the transistor of the power stage (npn in the considered example). The collector of power transistor T2 is connected to the amplifier input 2, whilst the emitter of said transistor is connected to the base of the power transistor T1. The base 8 of transistor T2 receives a current of intensity i making it possible to control the value of the current I flowing in load Z. It is assumed that the supply terminal 4 of generator G is a positive voltage terminal, whilst terminal 5 is a negative voltage terminal.
In this known type of current amplifier, the power and control transistors are of the same type. This type of amplifier has serious disadvantages. When the power transistor T1 is in the saturated state, the potential drop between the input and output terminals 2, 3 of the amplifier (potential drop between the emitter and collector of power transistor T1) is greater than that normally appearing between the emitter and the collector of said transistor when its base is not connected to another transistor for forming a Darlington circuit. Thus, this type of amplifier cannot function correctly if it is wished to use is as a relay.
This potential drop becomes even greater when several Darlington stages are connected in cascade, as is diagrammatically shown in exemplified form in FIG. 2. The same elements carry the same references in FIG. 2 as in FIG. 1. In this known circuit amplifier 1, whose input and output terminals are designated 2, 3 comprises a power stage 6 constituted by a power transistor T1, whose collector is connected to the input 2 (positive terminal 4 of generator G) and whose emitter is connected to load Z, itself connected to the negative terminal 5 of generator G. This amplifier also comprises a control stage 7 constituted by a Darlington circuit with two transistors T2, T3 connected in cascade. In this type of circuit, the potential drops between the emitter and the base of each transistor of the control stage are added to one another, so that between the base and collector of the power stage transistor T1 there is a voltage at least equal to the sum of these potential drops. Thus, transistor T1 cannot function at its saturation maximum. As the largest fraction of the current I supplying load Z passes through transistor T1 and as the Joule effect in the transistor T1 is equal to the product of the current passing through it by the emitter-collector voltage drop, in the junctions of transistor T1 there is a considerable Joule effect for high values of current I.
Moreover, in a Darlington circuit, the base-emitter potential difference of the power transistor T1 is reduced as a result of the supplementary voltage drop introduced by control transistor T2 and is further reduced by the presence of transistor T3 (and transistors of preceding stages, if applicable). Thus, the power transistor is not generally well saturated, so that this circuit, like the preceding one cannot function correctly as a relay.
FIG. 3 diagrammatically shows another known type of current amplifier 1 able to serve as a relay. This amplifier has an input 2 and an output 3, the latter being connected to a first terminal of a load Z. The current is supplied by a generator G having a first positive supply terminal 4 connected to the amplifier input 2 and a second negative terminal 5 connected to another terminal of load Z. This amplifier comprises a power amplifier stage 6 having a bipolar transistor T1 of a first type (e.g. npn). The collector of this transistor is connected to the amplifier input 2 and its emitter is connected to the amplifier output 3. This amplifier also comprises a stage 7 for controlling the transistor of the power stage, said control stage being constituted by at least one bipolar control transistor T4. This transistor T4 is of a second type (e.g. pnp), opposite to the first type. The emitter of transistor T4 is connected to the amplifier input 2 and the collector of said transistor is connected to the base 10 of power transistor T1. The base of control transistor T4 receives a control current i, which determines the intensity of the supply current I of load Z. The impedance of the amplifier stage varies in the reverse direction of the value of the control current, as will be shown hereinafter. In this circuit, each transistor is supplied by the entire potential difference between input 2 and output 3 of the amplifier, less a single emitter-base voltage drop for all the control transistors. Thus, in the case of FIG. 3, the emitter-base potential difference of power transistor T1 is deducted from the potential difference supplying the emitter-collector junction of control transistor T4. Said circuit type can function as a relay. However, the operation of the power transistor under saturation conditions is not perfect.
FIG. 4 diagrammatically shows a variant of the preceding circuit, which can also serve as a relay. The same elements carry the same references in both FIGS. 4 and 3. The amplifier or relay 1 here comprises a power amplifier stage 6 constituted by an e.g. npn transistor T1. The amplifier also comprises a control stage 7 of the transistor T1 of the power stage. The control stage 7 here comprises two transistors T5, T4 of alternate types. Transistor T4 is of the pnp type (because transistor T1 is e.g. of the npn type), whilst transistor T5 is of the npn type. The collector of transistor T4 is connected to the base of transistor T1 and the collector of transistor T5 is connected to the base of transistor T4. The emitter of the transistor T4 is connected to the positive terminal 4 of generator 5, and the emitter of this transistor T5 is connected to the output 3 of the amplifier. This type of circuit prevents the accumulation of the emitter-base voltage drops observed in the aforementioned Darlington circuit. However, as in the circuit of FIG. 3, the power transistor does no operate perfectly under saturation conditions.